Investigating CSI – Background material Table of Contents I ...

enable much more rapid, inexpensive analysis using many more probes, and
raising the odds against coincidental matches.
What are some of the DNA technologies used in forensic investigations?
Restriction Fragment Length Polymorphism (RFLP)
RFLP is a technique for analyzing the variable lengths of DNA fragments that
result from digesting a DNA sample with a special kind of enzyme. This enzyme,
a restriction endonuclease, cuts DNA at a specific sequence pattern know as a
restriction endonuclease recognition site. The presence or absence of certain
recognition sites in a DNA sample generates variable lengths of DNA fragments,
which are separated using gel electrophoresis. They are then hybridized with
DNA probes that bind to a complementary DNA sequence in the sample.
RFLP is one of the original applications of DNA analysis to forensic investigation.
With the development of newer, more efficient DNA-analysis techniques, RFLP is
not used as much as it once was because it requires relatively large amounts of
DNA. In addition, samples degraded by environmental factors, such as dirt or
mold, do not work well with RFLP.
PCR Analysis
PCR (polymerase chain reaction) is used to make millions of exact copies of
DNA from a biological sample. DNA amplification with PCR allows DNA analysis
on biological samples as small as a few skin cells. With RFLP, DNA samples
would have to be about the size of a quarter. The ability of PCR to amplify such
tiny quantities of DNA enables even highly degraded samples to be analyzed.
Great care, however, must be taken to prevent contamination with other
biological materials during the identifying, collecting, and preserving of a sample.
STR Analysis
Short tandem repeat (STR) technology is used to evaluate specific regions (loci)
within nuclear DNA. Variability in STR regions can be used to distinguish one
DNA profile from another. The Federal Bureau of Investigation (FBI) uses a
standard set of 13 specific STR regions for CODIS. CODIS is a software program
that operates local, state, and national databases of DNA profiles from convicted
offenders, unsolved crime scene evidence, and missing persons. The odds that
two individuals will have the same 13-loci DNA profile is about one in one billion.
Mitochondrial DNA Analysis
Mitochondrial DNA analysis (mtDNA) can be used to examine the DNA from
samples that cannot be analyzed by RFLP or STR. Nuclear DNA must be
extracted from samples for use in RFLP, PCR, and STR; however, mtDNA
analysis uses DNA extracted from another cellular organelle called a
mitochondrion. While older biological samples that lack nucleated cellular
material, such as hair, bones, and teeth, cannot be analyzed with STR and
RFLP, they can be analyzed with mtDNA. In the investigation of cases that have
gone unsolved for many years, mtDNA is extremely valuable.
All mothers have the same mitochondrial DNA as their daughters. This is
because the mitochondria of each new embryo comes from the mother's egg cell.
The father's sperm contributes only nuclear DNA. Comparing the mtDNA profile
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